(1) Field of the Invention
The present invention relates in general to a method and a device for flying safely at low altitude in an aircraft in spite of wire and non-wire obstacles, by making use of detector means.
(2) Description of Related Art
More particularly, and in non-limiting manner, the detector means are of the radar or laser or stereoscopy telemeter type. It is recalled that remote detection by laser, known as light detection and ranging (lidar) makes use of laser light returned to the emitter.
An object is detected by measuring the time delay between emitting a signal and detecting the reflected signal, the signal being constituted by radio waves when using radar or by light rays when using lidar.
From images delivered by detector means producing plots of individual echoes, it is known to obtain a terrain elevation database for the zone observed by the detector means. The database includes all of the relief and the obstacles.
Nevertheless, it is observed that failure to detect cables or other suspended wire obstacles lies behind numerous flying accidents, and reduces the domain in which aircraft, and in particular helicopters, can fly safely when close to the ground.
Patents FR 2 736 149 and U.S. Pat. No. 5,762,292 already make proposals for a system that recognizes structures that present rectilinear portions in an image delivered by a sensor on board a flying machine, by making use of a parametric transform (Hough transform) of a portion of the image.
The Hough transform, described in U.S. Pat. No. 3,069,654, serves to detect a set of aligned points in an image.
U.S. Pat. No. 5,296,909 proposes detecting the presence of cables by using a scanning laser telemeter (lidar) that delivers plots, where each plot corresponds to a point in three-dimensional space characterized by its three spatial coordinates, specifically spherical coordinates given as elevation, relative bearing, and range: the telemeter sends laser pulses that make it possible, by measuring their round-trip times, to obtain points that are positioned in three-dimensional (3D) space. The echoes are filtered. A set of parameters is determined by the Hough transform for all possible groups of filtered echoes. Clusters of points in parameter space are identified, and the position of a cable is determined by the inverse Hough transform.
Proposals are also made in U.S. Pat. No. 6,747,576 to detect the presence of electricity lines by forming a cloud of measurement points in a terrestrial frame of reference on the basis of data delivered by a remote detector sensor and data from a navigation system, with measurement points that represent the ground being eliminated therefrom. The method then includes a search for straight lines amongst the projections of the measurement points onto the horizontal plane, by using two successive Hough transforms: a “pure” transform using a delta function (or Dirac function), followed by a “fuzzy” transform in which the delta function is replaced by a Gaussian distribution. Thereafter, a search is made for catenaries in each vertical plane containing one of the straight lines found in that way, this search also making use of two successive Hough transforms.
In order to search for a catenary corresponding to the equation:[z=a*cos h((λ−b)/a)+c]for each measurement point of each vertical plane, and for each possible value of a catenary parameter a, a two-dimensional Hough transform is calculated (in the b and c parameter space) for catenaries passing via that point.
The document “Automatic extraction of vertical obstruction information from interferometric SAR elevation data” by Donald Woods et al. (IEEE publication: IGARSS 2004 Congress) provides a method of calculating the height and the location of vertical obstacles from a digital terrain model enabling high points to be extracted.
Those various devices are effective. Nevertheless, active sensors for detecting obstacles are limited in particular by the technique used for detecting wire obstacles, since it is not capable of detecting wire obstacles from below a threshold angle of incidence for the emitted signal relative to the wire obstacle, where said angle of incidence is about 15° with radar and about 60° with lidar. As from that angle of incidence, reflection becomes specular and it is no longer possible to detect cables.
Present-day terrain elevation databases do not make it possible to fly any closer to the ground since there is no guarantee that wire obstacles will be detected. The pilot is thus obliged to fly higher above the ground so as to leave a safety margin.
Systems using obstacle databases exist, but they are not complete, they are not guaranteed by their constructors, and they are provided for information purposes only.
It should be observed that the state of the art also includes the following documents: US 2007/171094, US 2003/225489, FR 2 895 098, US 2004/267413, and the article by M. Zhao et al.: “A method to identify flight obstacles on digital surface model” (Tsinghua Science and Technology, Tsinghua University Press, Beijing, China, Vol. 10, No. 3, Jun. 1, 2005).